Transition absorption

The longest wavelength absorption transition for ethene calculated by HyperChem using PM3 is 207 nm, which compares favorably with the experimental value of 190-200 nm. After you compute an electronic spectrum with HyperChem, you can use the table below to assign computed transitions and qualitatively assess the accuracy of the computation. ... 

Kosower made the first use of this phenomenon for measuring solvent polarity. The model process is the absorption transition of l-ethyl-4-carbomethoxy-pyridinium iodide, 7 ... 

Energy levels for the hydrogen atom and some of the transitions that occur between levels. Upward arrows represent absorption transitions, and downward arrows represent emissions. 

Interestingly enough, it is possible to study these systems also by emission spectroscopy. The results for In(III) are conspicious (see Table 1). Figure 7 gives the luminescence spectra of LajTaO Clg In(III) to illustrate the type of spectra [48] we are dealing with broad bands the emission is strongly Stokes-shifted relative to the absorption transition. 

The identification of xanthophylls in vivo is a complex task and should be approached gradually with the increasing complexity of the sample. In the case of the antenna xanthophylls, the simplest sample is the isolated LHCII complex. Even here four xanthophylls are present, each having at least three major absorption transitions, 0-0, 0-1, and 0-2 (Figure 7.4). Heterogeneity in the xanthophyll environment and overlap with the chlorophyll absorption add additional complexity to the identification task. No single spectroscopic method seems suitable to resolve the overlapping spectra. However, the combination of two spectroscopic techniques, low-temperature absorption and resonance Raman spectroscopy, has proved to be fruitful (Ruban et al., 2001 Robert et al., 2004). 

The experimental observations of red shifts of the UV absorptions tails with increase in silicon dimensionality were corroborated by ZINDO-calculated spectra comparing linear polysilane, network polysilyne, crystalline cluster, and amorphous cluster structures, which showed respectively lowest absorption transition energies of 5.38 eV (230.4 nm), 4.60eV (269.5nm), 4.57eV (271.2nm), and 2.46eV (503.9nm), as shown in Figure 57.362... 

The concept of transition moment is of major importance for all experiments carried out with polarized light (in particular for fluorescence polarization experiments, see Chapter 5). In most cases, the transition moment can be drawn as a vector in the coordinate system defined by the location of the nuclei of the atoms4 therefore, the molecules whose absorption transition moments are parallel to the electric vector of a linearly polarized incident light are preferentially excited. The probability of excitation is proportional to the square of the scalar product of the transition moment and the electric vector. This probability is thus maximum when the two vectors are parallel and zero when they are perpendicular. 

For 7i —> 7i4 transitions of aromatic hydrocarbons, the absorption transition moments are in the plane of the molecule. The direction with respect to the molecular axis depends on the electronic state attained on excitation. For example, in naphthalene and anthracene, the transition moment is oriented along the short axis for the So —> Si transition and along the long axis for the S0 —> S2 transition. Various examples are shown in Figure 2.3. 

Fig. 2.3. Examples of molecules with their absorption transition moments.

There are two major selection rules for absorption transitions ... 

Most chromophores absorb light along a preferred direction1 (see Chapter 2 for the definition of absorption transition moment, and for examples of transition moments of some fluorophores, see Figure 2.3), depending on the electronic state. In contrast, the emission transition moment is the same whatever the excited state reached by the molecule upon excitation, because of internal conversion towards the first singlet state (Figure 5.2). 

If the incident light is linearly polarized, the probability of excitation of a chro-mophore is proportional to the square of the scalar product MA.E, i.e. cos2 0A, 8 being the angle between the electric vector E of the incident light and the absorption transition moment MA (Figure 5.2). This probability is maximum when E is parallel to MA of the molecule it is zero when the electric vector is perpendicular. 

The absorption transition moment is not in a single direction for some molecules whose symmetry is (benzene),... 

For a rod-like probe with its absorption transition moment direction coinciding with the long molecular axis, the rotational motion in this potential well is described by the diffusion coefficient D. The decay of the autocorrelation functions is then shown to be an infinite sum of exponential terms ... 

Figure 8.9. Calculated excitation spectra for different orientations of the absorption transition moment 6m for the same particle as in Figure 8.4.

Keywords Absorption In silico Mixing tank Maximum absorbable dose Mass balance approach Compartmental absorption Transit models... 

The upward arrows in the figure indicate the pumping channels to various high energy levels by flashlamp (0.5 /tim) or semiconductor lasers (0.8 /rm), where Nd + ions display strong absorption transitions. The downward arrow indicates the widely used laser emission at 1.06 /xm, associated with the -> " ln/2 transition. In addition, laser action is also generally possible from the same " F3/2 level to the " 19/2 state at around 0.9 /xm and to the 113/2 state at around 1.3 /xm. 

In the spirit of the adiabatic approximation, the transitions between two vibrational states (belonging to initial and final electronic states) must occur so rapidly that there is no change in the configurational coordinate Q. This is known as the Frank Condon principle and it implies that the transitions between i and / states can be represented by vertical arrows, as shown in Figure 5.12. Let us now assume our system to be at absolute zero temperature (0 K), so that only the phonon level = 0 is populated and all the absorption transitions depart from this phonon ground level to different phonon levels m = 0, 1, 2,... of the excited state. Taking into account Equation (5.25), the absorption probability from the = 0 state to an m state varies as follows ... 

F ure 6.16 A typical absorption spectrum of a trivalent rare earth ion (not corresponding to any specific ion) in a crystal. A generic / —> / absorption transition, with an average frequency of mo, has been marked and shaded (see the text). 

Since our system is in equilibrium, the number of absorption transitions i f per unit time must be equal to the number of emission transitions / / per unit time. Considering that the light-matter interaction processes described in Chapter 2 (Figure 2.5) are taking place, in equilibrium the rate of absorption must be equal to the rate of (stimulated and spontaneous) emission. That is ... 

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